In order to improve the abrasion resistance and the scratch resistance of paints, various additives are added, which include especially silica powders and/or polyamide powders.
However, the use of silica powders is limited, on the one hand, because of the sedimentation which takes place within the paints into which they are introduced and, on the other hand, because of their low resistance to crayoning (ability of a paint-covered surface to be marked after being rubbed with an object, especially metallic; this phenomenon being all the more visible when the paint is light in tint). To improve the dispersion of the silica powders and to limit their sedimentation within paints, this being in order to obtain better stability of the said paints, it has been proposed to modify the silica powders, for example by addition of polyethylene waxes, but the mixture is tricky to produce and this technical solution does not produce any improvements insofar as the other weak points referred to above are concerned.
Polyamide (PA) powders exhibit excellent abrasion and scratch resistance properties but their presence in paints generally does not improve the resistance to crayoning either. In addition, milling polyamide resins to the desired particle size--of the order of a few tens of .mu.m--is sometimes found to be tricky when the said PA powders are obtained by polycondensation. PA powders of the desired particle size can be obtained directly without subsequent grinding, by anionic polymerization and especially according to the process described in Patent Application FR 2 619 385 or in Patent EP 303 530.
The paints containing silica powder and polyamide powder, described in Kansai Paint Co's Patent Application JP62074970 offer a compromise which is correct but still insufficient insofar as the abrasion, scratch and crayoning resistance is concerned.